It solely depends on what modifications you do to get there.

Like a factory supercharged car with a smaller blower pulley for more boost. This will still pass a sniffer test. Nitrous will still pass the sniffer test. Those 2 examples will not change the exhaust from what the sniffers tests.

Camshafts and turbos are more of a challenge to pass the sniffer. If you get more overlap in a cam. This will definately pump up the polutants in the exhaust. Turbos change the hot side of an exhaust. Many challenges there.

I'm not saying it can't be done. In fact many people do it and have done it. It's just there are easier ways than others.

With the area you live in being the deciding factors. For example. Here in Ontario. Most fast cars are old school. No emissions test required. These are usually big cubic inch, racing cylinder head monsters.

 

Well we dont have a sniffer test here, they just plug in to the OBD port.

 

^same here in MA. I have a o/r x pipe and I pass becasue 1)I've gone to the same guy since I first got a car 2) He donen't give a **** lol 3) I have MIL's so I have no CEL's which means with him only testing through my OBD port, I pass.

If I still had sniffer tests I'd be screwed. Hell the car SMELLS dirty just sitting there idling lol But that's where the guy's lack of caring comes in lol.